The invention concerns the field of the demineralization of sweet whey with an electrodeionization process.
Demineralized whey, whether liquid or in powder form, constitutes the main component of products for infants and dietetic food products, in particular milks adapted to mother's milk. Demineralized whey also has other applications, for example as ingredients for the replacement of skimmed milk in confectionery-chocolate manufacture or in the manufacture of reconstituted milks.
The most effective known processes for demineralizing whey are electrodialysis and ion exchange, which are applied separately or in combination. In electrodialysis, ionized salts in solution in the whey migrate under the effect of an electric field through membranes which are selectively permeable to cations and to anions and are eliminated in the form of brine. In ion exchange, the ionic equilibrium is used between a resin as the solid phase and the whey to be demineralized as the liquid phase, the ions being adsorbed on the resin of the same nature during the saturation phase, and the resins are then regenerated.
For reasons of productivity, these two techniques are advantageously combined in a two-step process, electrodialysis ensuring an initial demineralization to approximately 50-60% and ion exchange, preferably multi-stage with successive weakly cationic and strongly cationic resins, achieving final demineralization to approximately 90-95%, as is described for example in U.S. Pat. No. 4,803,089.
These processes have the disadvantages that the ionic exchange stage requires large quantities of regenerating chemicals and consumes a large amount of water and that electrodialysis cannot be used beyond a degree of demineralization &gt;60% due to its large electrical energy demand.
Electrodeionization, which is for example the subject of U.S. Pat. No. 4,632,745 or U.S. Pat. No. 5,120,416, carries out deionization continuously in the treatment of water by combining electro-dialysis and ion exchange in a single module, which has the advantage of low consumption of water and energy and eliminates the necessity of chemically regenerating the resins.
The electrodeionization technique consists of circulating the water to be demineralized through an assembly of cells in parallel delimited by cationic and anionic semi-permeable membranes and containing a mixture of resin beads, referred to as dilution compartments, these dilution compartments being separated from each other and their assembly being separated from the outside by spacers, forming compartments referred to as concentration compartments, delimited by anionic and cationic semi-permeable membranes, the complete assembly being placed between a cathodic compartment and an anodic compartment connected to an electrical supply. Wash water is circulated through the concentration spaces, which enables the ions which concentrate there on account of their polarity, to be eliminated in the form of effluent, by migrating through the membranes under the effect of the electric field from the dilution compartments to the concentration compartments.
Unlike the case of electrodialysis, resin beads loaded with adsorbed ions maintain a sufficient electric conductivity in the dilution compartments throughout the demineralization process. Moreover, it is not necessary to regenerate the resin beads, since the sites saturated with cations and anions are exchanged progressively with H.sup.+ and OH.sup.- under the effect of the electric field.
In the process according to U.S. Pat. No. 4,632,745, resin beads are incorporated in a fixed manner in the dilution compartments whereas in the process according to U.S. Pat. No. 5,120,416, the beads are movable and it is possible to introduce them into the dilution compartments and to extract them from the compartments by circulation in the form of a suspension. In these known processes applied to water, the resins are present in a mixed bed of beads of a strongly cationic and strongly anionic type.